Supporting Information
A Double-Layered Composite for Lightning Strike Protection via Conductive
and Thermal Protection
Qianshan Xiaa, Zhichun Zhanga, Hao Meia, Yanju Liub, Jinsong Lenga,*
a National Key Laboratory of Science and Technology on Advanced Composites in
Special Environments, No. 2 YiKuang Street, Science Park of Harbin Institute of
Technology (HIT), Harbin, 150080, P.R. Chinab Department of Aerospace Science and Mechanics, No. 92 West DaZhi Street, Harbin
Institute of Technology (HIT), Harbin, 150001, P.R. China
Buckypaper (BP) was prepared by single-wall carbon nanotubes (SWCNTs, TNSR,
Chengdu Organic Chemicals Co., Ltd.) and surfactant (Triton X-100, Aladdin
Chemical Reagent Co., Ltd.) [1]. The size of the as-prepared BP was 370370 mm2
and its areal density was 4 mg/cm2. The as-prepared BP was treated at 350 oC for 2 h,
to remove the residual surfactant. One side of a piece of BP was adhered with a
plastic film completely as a cathode and a graphite plate was placed in electrophoretic
deposition (EPD) bath as an anode. Two electrodes kept parallel and their distance
was around 20 mm. To prepare the electroplating solution, 55 g silver nitrate (AgNO3,
Shanghai Shiyi Chemicals Reagent Co., Ltd.) was dissolved in 5000 mL deionized
water with stirring, then 2.5 g magnesium nitrate (Mg(NO3)2, Damao Chemical
Reagent Co., Ltd.) was added into the AgNO3 solution [2]. After mechanical stirring
for 30 min, the mixed solution was transferred into the EPD bath, and the anode was
overwhelmed by the liquid level. During the EPD process, the applied DC voltage
was 5 V and the deposition time was 240 s. Finally, the as-prepared silver modified
buckypaper (SMBP) was dried and peeled from the plastic film. The size of the
SMBP was about 370370 mm2.
The carbon fiber/phenol-formaldehyde (CF/PF) prepreg was fabricated via the resin
transfer molding (RTM) process, as shown in Figure S1. The carbon fiber woven
fabric (CF, Weihai Guangwei Composites Co., Ltd., W-1021) was cut into 370370
mm2. The CF was placed on the mold surface. Paved flow media on the CF surface
and sealed the mold as a vacuum bag. Moreover, fixed two rubber pipes on the
vacuum bag for exhausting air and importing resin. Connected one pipe to a vacuum
pump and exhausted the residual air in the vacuum bag for 5 min. To inject the PF
solution (Institute of Chemistry, Chinese Academy of Sciences, Content 75 wt%), put
another pipe into a beaker that held some PF solution and kept the pump working.
Figure S1. The schematic diagram of the fabrication process of the CF/PF prepreg
Carbon fiber reinforced polymer (CFRP), (Cu mesh/carbon fiber reinforced polymer)
Cu/CFRP, SMBP/CFRP and SMBP-CF/PF-CFRP test specimens were fabricated by
an autoclave through vacuum hot pressing. CFRP prepregs (Weihai Guangwei
Composites Co., Ltd., T300 125 g) were cut into 370370 mm2. 32 layers of CFRP
prepregs were paved together to form a CFRP laminate and the stacking sequence was
[0°/90°]16S. The CFRP composite was cured in a vacuum bag. Cu mesh (Dexmet
Corporation, 142gsm) and SMBP were placed on the outmost layers of two CFRP
laminates containing 32 layers of prepregs, respectively. In addition, one layer of the
CF/PF prepreg was firstly paved on the CFRP laminate surface which contained 31
layers of prepregs, then the SMBP was paved on the CF/PF surface. Subsequently,
Cu/CFRP, SMBP/CFRP and SMBP-CF/PF-CFRP laminates were also cured under
the same conditions as the CFRP composite. The schematic profile of preform
fabrication of SMBP-CF/PF-CFRP laminate was shown in Figure S2a). Temperature-
time and pressure-time curves used in the hot-pressing process were shown in Figure
S2b).
Figure S2. a) Schematic profile of the preform fabrication and b) temperature and pressure curves used in the curing process
The sample was positioned in the setup before the simulated lightning strike (LS) test
as shown in Figure S3a), and the simulated LS process was shown in Figure S3b) and
Video S1.
Figure S3. a) The sample in the setup before the simulated LS test and b) the composite panel during the simulated LS test
Figure S4 demonstrates the Raman spectrum of the BP for analyzing structural defects
of CNTs caused by the EPD. The ID/IG value of the BP is 0.023.
Figure S4. Raman spectrum of the BP
Silver (Ag) particles cover part of intrinsic pores of CNT networks and generate some
new pores through particles stacking on the BP surface. Thus, it is necessary to
investigate pore features of the SMBP for the preparation of the CFRP matrix
composite. The pore diameter distribution of the SMBP is displayed in Figure S5a).
Its pore volume peak appears at 31.3 nm and range of the pore diameter distribution is
1.8-52.2 nm. According to the International Union of Pure and Applied Chemistry’s
(IUPAC) classification [3], the SMCNP can be considered as a typical mesoporous
material. In addition, the spreading ability of the resin on the SMBP surface could be
influenced by Ag particles adding. Contact angles of epoxy and PF resin droplets on
two sides of SMBP were observed, for evaluating their spreading abilities on SMBP
surfaces. As shown in Figure S5b) and c), contact angles of epoxy droplets on non
EPD and EPD sides of the SMBP at room temperature are 15.9° and 25.3°,
respectively. While, contact angles of PF droplets on non EPD and EPD sides of the
SMBP are 19.8° and 28.6° at room temperature in Figure S5d) and e), which are little
larger than epoxy droplets. It illustrates that both the two kinds of resins can easily
spread on two sides of the SMBP. Furthermore, viscosity of both two resins will
reduce during the preparation processes of CFRP matrix composites, owing to
ambient temperature increasing. It means that SMBPs can be permeated by the rich
epoxy and PF resins to form the composite with stable structure, and its resin
compatibility is slightly affected by the EPD process.
Figure S5. a) The pore size distribution of the SMBP; contact angles of epoxy resin droplets on b) non EPD and c) EPD sides of the SMBP; contact angles of PF resin droplets on d) non EPD and e) EPD sides of the SMBP.
At the lightning attachment point, the huge Joule heat caused by a lightning current of
100 kA will generate the ultra-high surface temperature, which will cause pyrolysis of
the CFRP composite. Pyrolysis processes of two protective layers were studied,
including SMBP and CF/PF. Figure S6b) shows that the whole decomposition process
of the SMBP contains three main stages. The first stage starts at 200 oC and its
decomposition peak appears at 237 oC. It attributes to decomposition of the residual
AgNO3. Decomposition products include silver (Ag), oxygen (O2) and nitrogen
dioxide (NO2). The decomposition temperature of the Ag2O is 250 oC, thus the
elemental silver will not oxidize anymore during the subsequent heating process. The
onset decomposition temperature of the residual Mg(NO3)2 is 290 oC and its
decomposition rate reaches the maximum at 410 oC. The Mg(NO3)2 decomposes and
generates magnesium oxide (MgO), oxygen (O2) and nitrogen dioxide (NO2). When
the temperature reaches 450 oC, pyrolysis of CNTs starts. The temperature goes up to
600 oC and the decomposition rate reaches the maximum. When the temperature rises
to 665 oC, the mass of the remnant no longer changes. The main remnant is the
elemental silver, a few residual MgO and oxides of residual catalysts of CNTs.
Compared with Figure S6a), the decomposition temperature of CNTs of the SMBP
increases and its decomposition rate is slower. In addition, Ag content of the SMBP is
about 25.22%, according to TGA curves of BP and SMBP. It indicates that the SMBP
possesses low areal density. TGA and DTG curves of the CF/PF composite layer were
analyzed to study its pyrolysis process. The decomposition process of the CF/PF
composite can be divided into three main stages (Figure S6c). In the initial stage, the
CF/PF composite starts to degrade at 118 oC and its peak appears at 177 oC. It
corresponds to volatilization of residual curing agents and raw materials without
reaction. The mass loss of the first stage is less, which is only about 1.7 wt%. The
pyrolysis of PF resin starts, when the temperature goes up to 319 oC. The maximum
rate occurs at 594 oC that corresponds to the pyrolysis of the PF resin in the second
stage. Its pyrolysis products mainly contain water (H2O), carbon monoxide (CO), and
carbon dioxide (CO2), acetylene (C2H4) [4].When the temperature rises to 640 oC, the
PF resin decomposes completely and the CF starts to pyrolyze. The decomposition
rate of CF reaches maximum at 770 oC. When the temperature increases beyond 806 oC, there is no residual CF [5].
Figure S6. TGA and DTG curves of a) BP, b) SMBP and CF/PF composite.
Lightning strike protection (LSP) performance of the LSP structure can be directly
evaluated by the residual strength of the composite after the LS. In term of the ASTM
D7137, the residual strength values of the above-mentioned composite panels were
obtained through compressive tests [6]. The neat CFRP panel without the LS, as the
control specimen, was cut into the rectangular specimen as shown in Figure S7a).
Compressive specimens of neat CFRP, Cu/CFRP, SMBP/CFRP and SMBP-CF/PF-
CFRP panels after LS tests were displayed in Figure S7b), c), d) and e), respectively.
Compressive strength values of control specimen, neat CFRP specimen, Cu/CFRP
specimen, SMBP/CFRP specimen and SMBP-CF/PF-CFRP specimen are 292.1 MPa,
176.58 MPa, 280.68 MPa, 265.05 MPa and 284.08 MPa, respectively. In contrast with
the compressive strength of the control specimen, retention rates of compressive
strength of neat CFRP, Cu/CFRP, SMBP/CFRP and SMBP-CF/PF-CFRP composites
after LS tests are 60.45%, 96.09%, 90.74% and 97.25%, respectively. The residual
strength of the SMBP-CF/PF-CFRP composite is 60.88% higher than that of the
CFRP composite without protection and 1.21% higher than the Cu/CFRP composite.
It indicates that the SMBP-CF/PF composite LSP structure can effectively prevent
damage of the CFRP matrix caused by the LS.
Figure S7. Compressive specimens of a) control group, b) neat CFRP, c) Cu/CFRP, d) SMBP/CFRP and e) SMBP-CF/PF-CFRP composites, f) compressive strength values and retention rates of specimens.
During the LS process, fuel tank, engine and other parts of the aircraft need strictly
limiting the temperature variation of the non LS side of the composite, thus it is
significant to study the maximum temperature difference of the LSP composite.
Figure S8a), b), c) and d) present maximum temperatures of non LS sides of CFRP,
Cu/CFRP, SMBP/CFRP and SMBP-CF/PF-CFRP panels during simulated LS tests,
respectively. When lightning strikes the CFRP surface, more LS energy is conducted
along the through-thickness direction of the composite and causes serious depth
damage, owing to its poor conductivity. During the LS process, the maximum
temperature variation of the non LS face of the CFRP panel was 114.8 oC, as
displayed in Figure S8a). It illustrates that the ability of the CFRP composite to
dissipate the LS energy is too poor. Adding Cu mesh as a LPS layer can obviously
enhance the conduction ability along the composite surface for the LS energy and
reduce Joule heat conducting along the through-thickness direction of the CFRP
matrix. As shown in Figure S8b), the maximum temperature variation of the non LS
side of the Cu/CFRP panel was 51.1 oC. For the SMBP/CFRP composite, the
maximum temperature variation of its non LS face was 52.8 oC during the LS test
(Figure S8c). Because the LS energy is conducted on the SMBP surface by the plane
conduction and the ability of the SMBP to dissipate the LS energy is increased by the
re-solidified silver frame forming. It reduces the LS energy transferring into its CFRP
matrix. Figure S8d) displays that the maximum temperature variation of the non LS
face of the SMBP-CF/PF-CFRP panel was only 44.9 oC during the LS process, which
was lower than that of CFRP and Cu/CFRP panels. Besides electrical protection of the
SMBP, the PF resin decomposes rapidly and absorbs part of Joule heat under the high
temperature. The carbon layer generated by pyrolysis of the PF resin radiates residual
heat to the environment and can reduce the surface temperature obviously. It proves
that introduction of the SMBP-CF/PF LSP structure can effectively reduce the LS
energy transferring to the inner CFRP matrix.
Figure S8. Infrared thermal images of maximum temperatures of non LS sides of a) neat CFRP, b) Cu/CFRP, c) SMBP/CFRP and d) SMBP-CF/PF-CFRP composite panels during LS tests.
The surface temperature of the composite cannot be acquired by the state of the art
during the LS process, owing to ultra-short time and ultra-high temperature. The heat
transmission process of the CF/PF layer is studied through the numerical simulation
method, for verifying the structural reasonability of the SMBP-CF/PF-CFRP
composite. The heat transmission process of the CF/PF layer and the CFRP matrix as
a system is calculated with the finite element modeling (FEM) simulation by the
COMSOL Multiphysics software [7]. Thermal protection of the LS is similar to the
ablative protection, thus the numerical model of the ablative protection is used to
analyze the thermal transmission process of the CF/PF layer during the LS process
[8]. The heat transmission is only considered along the thickness-direction of the
system and the heat transferring along the other directions is ignored for simplifying
the mathematical model as a quasi one-dimensional model, because the damage depth
will affect the residual strength of CFRP matrix composite seriously. Thus, the quasi
one-dimensional model can illustrate the heat-protective function of the CF/PF layer
and analyze its LSP mechanism. The Joule heat that was generated by the LS current
causes rapid increase of the surface temperature of the CF/PF. The formation of the
ultra-high temperature difference between the top and bottom surfaces of the CF/PF
layer leads to the heat transferring from the high-temperature zone to the low-
temperature zone. The transmission mode is mainly depended on the heat conduction.
PF resin will rapidly pyrolyze through the endothermic reaction under the high
temperature. The pyrolytic gas flow will cause the heat convection, which can be
ignored resulting from small thickness of the CF/PF layer and short time of the LS
process. In addition, top surface of the CF/PF layer after the carbonization will
spontaneously radiate heat to the environment. Therefore, the heat radiation
phenomenon should be considered as the boundary of the FEM model [9].
Based on the above analysis, the mathematical model of the CF/PF layer is described
as following. During the LS, the thermal transmission process of the CF/PF layer
follows the law of energy conservation and its equation under the thermal loading
condition can be described as [9]:
ρC ∂ T∂ t
=∇ ∙ ( k∇T )+ ∂ ρ∂ t
∆ H (S1)
where , C, T, t, k are density, specific heat capacity, thermodynamic temperature,
time of the heat transmission process and thermal conductivity of the CF/PF layer in
Equation (S1), respectively. H is the energy consumption of resin per unit mass
pyrolysis.
To simplify calculation, density, specific heat capacity and thermal conductivity are
obtained by weighted averages of thermophysical properties and volume fractions of
component phases. Then, equations are described as following:
ρ=ε f ρf +εm ρm (S2)
C=ε f C f +ϵm Cm (S3)
k=ε f k f +εm km (S4)
where f and m as subscripts in Equation (S2), (S3) and (S4) stand for carbon fiber and
PF resin of the CF/PF layer, respectively.
Volume fractions of carbon fiber and PF resin of the CF/PF layer are expressed as ε f
and ε m, respectively. The relationship betweenε f and ε m can be written as:
ε f +εm=1 (S5)
When the temperature reaches the decomposition temperature, PF resin will pyrolyze.
The density variation of the PF resin can be expressed by the Arrhenius law [10]:
∂ ρm
∂ t=
−J o
ρb( ρm−ρm
∞ )exp (−EA
Rθ) (S6)
where m, Jo, b,ρm∞, EA are density of the resin, pre-exponential factor, density of
the resin phase, density of the resin after decomposition and activation energy in
Equation (S6), respectively. R stands for the gas constant, which is 8.314 J/(molK).
During the pyrolysis process, specific heat capacity of the PF resin changes with the
temperature and can be described as [10]:
C ()=❑bC b❑b( )+❑P Cp❑p()
❑bCb+❑PC p (S7)
where C(), b, Cb, b(), p, Cp and p() mean specific heat capacity of the matrix,
density of the resin phase, specific heat capacity of the resin phase, temperature of the
resin phase, density of the pyrolytic phase, specific heat capacity of the pyrolytic
phase, temperature of the pyrolytic phase in Equation (S7), respectively.
Heat flow and energy radiating to the environment should be considered on the LS
side of the CF/PF layer. The boundary condition of LS side of the CF/PF layer can be
written as:
−k ∂ T∂
=qn−σε (T w4 −T ∞
4 ) (S8)
where qn and are heat flow cause by the LS and radiation coefficient of the pyrolysis
layer, respectively. is the Stephan-Boltzman constant, which is 5.6710-8 W/m2K4
[11]。
The initial condition is:
T 0t=0=293 K (S9)
During the LS process, the mode of heat transmission of the CFRP matrix is the same
as the CF/PF layer, thus equations and boundary condition of the CFRP matrix are
also the same as the CF/PF layer.
The surface temperature of the composite cannot be acquired by the state of the art
during the LS process, owing to ultra-short time and ultra-high temperature. The heat
transmission process of the CF/PF layer is studied through the numerical simulation
method, for verifying the possible mechanism of the CF/PF protective layer. The heat
transmission process of the CF/PF layer and the CFRP matrix as a system is
calculated with the finite element modeling (FEM) simulation by the COMSOL
Multiphysics software. Thermal protection of the LS is similar to the ablative
protection, thus the numerical model of the ablative protection is used to analyze the
thermal transmission process of the CF/PF layer during the LS process. The heat
transmission is only considered along the thickness-direction of the system and the
heat transferring along the other directions is ignored for simplifying the
mathematical model as a quasi one-dimensional model, because the damage depth
will directly affect the residual strength of the CFRP composite seriously. Thus, the
quasi one-dimensional model can illustrate the heat-protective performance of the
CF/PF layer and analyze its LSP mechanism. The Joule heat that is generated by the
LS current causes rapid increase of the surface temperature of the CF/PF. The
formation of the ultra-high temperature difference between the top and bottom
surfaces of the CF/PF layer leads to the heat transferring from the high-temperature
zone to the low-temperature zone. The transmission mode is mainly depended on the
heat conduction. PF resin will rapidly pyrolyze through the endothermic reaction
under the high-temperature. The pyrolytic gas flow will cause the heat convection,
which can be ignored resulting from small thickness of the CF/PF layer and short time
of the LS process. In addition, top surface of the CF/PF layer after the carbonization
will spontaneously radiate heat to the environment. Therefore, the heat radiation
phenomenon should be considered as the boundary of the FEM model. Based on
above analysis, the mathematical model of the CF/PF layer is described as Equation
(S1)-(S9) and the schematic diagram of the FEM model is shown in Figure S9a).
Figure S9. a) Schematic diagram of geometry and boundary of the FEM model, b) temperature-time curves of top and bottom surfaces of the CF/PF-CFRP system during the LS process in 30 s; the inset is temperature-time curves of top and bottom
surfaces of the CF/PF-CFRP system during the LS process in 0.6 s, c) temperature fields of the CF/PF-CFRP system at 0 s, 0.38 s, 0.4 s and 6.26 s.
The numerical model was solved by the COMSOL solver. Figure S9b) shows the
temperature of the CF/PF layer increases rapidly during the initial LS stage. Under the
high temperature, the PF resin pyrolyzes and generates gas and charring layer. During
the pyrolysis process, the PF resin absorbs some heat with the enthalpy change and
the charring layer radiates part of heat to the environment. The CF/PF layer can
effectively dissipate most of the Joule heat, but there is still residual heat transferring
into the CFRP matrix. Temperature fields of the simulating heat transmission process
are described as Figure S9c). The top surface temperature of the CF/PF layer reaches
the maximum at 0.38 s. The heat flux stops at 0.4 s and the top surface temperature
start dropping, owing to pyrolysis of the PF resin dissipating heat. The surface
temperature of the CFRP matrix rises more slowly and reaches the maximum
temperature at 0.44 s, due to thermal protection of the CF/PF layer. Then, continuous
pyrolysis of the PF resin leads to the temperature gradually falling. When the
temperature of the system drops below 600 K at 6.26 s, the PF resin will not
decompose. The system is only depended on the radiation to transfer the heat and
cooled to the initial temperature.
Table S1. Parameters of D, B and C* waveforms
D waveform B waveform C waveform
Ipeak/avg (kA) 100 2 0.4
AI (A2s) 0.25106 - -
Q (C) - 10 21
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